Merge bitcoin/bitcoin#26617: test: add extra_args to BitcoinTestFramework class

150340a test: remove unneeded extra_args code (josibake)
989a52e test: add extra_args to BTF class (josibake)

Pull request description:

  ## problem
  If you try to add `extra_args` when using `TestShell`, you will get the following error:

  ```python
  >>> import sys
  >>>
  >>> sys.path.insert(0, "/home/josibake/bitcoin/test/functional")
  >>>
  >>> from test_framework.test_shell import TestShell
  >>> test = TestShell().setup(num_nodes=2, extra_args=[[],['-fallbackfee=0.0002']])
  Traceback (most recent call last):
    File "<stdin>", line 1, in <module>
    File "/home/josibake/bitcoin/test/functional/test_framework/test_shell.py", line 41, in setup
      raise KeyError(key + " not a valid parameter key!")
  KeyError: 'extra_args not a valid parameter key!'
  >>>
  ```

  ## solution

  add `self.extra_args = None` so that `extra_args` is recognized as a valid parameter to be passed to `BitcoinTestFramework`

  ```python
  >>> import sys
  >>>
  >>> sys.path.insert(0, "/home/josibake/bitcoin/test/functional")
  >>>
  >>> from test_framework.test_shell import TestShell
  >>> test = TestShell().setup(num_nodes=2, extra_args=[[],['-fallbackfee=0.0002']])
  2022-12-01T11:23:23.765000Z TestFramework (INFO): Initializing test directory /tmp/bitcoin_func_test_sbwthbb_
  ```

ACKs for top commit:
  willcl-ark:
    re-ACK 150340a

Tree-SHA512: e6fa2a780a8f2d3472c322e8cdb00ec35cb220c3b4d6ca02291eb8b41c0d8676a635fbc79c6be80e3bb71d700a2501a4b73f762478f533ae453d492d449307bb

Author: MarcoFalke

test/functional/test_framework/test_framework.py

@@ -97,6 +97,7 @@ def __init__(self):
         self.chain: str = 'regtest'
         self.setup_clean_chain: bool = False
         self.nodes: List[TestNode] = []
+        self.extra_args = None
         self.network_thread = None
         self.rpc_timeout = 60  # Wait for up to 60 seconds for the RPC server to respond
         self.supports_cli = True
@@ -408,10 +409,7 @@ def setup_network(self):
 
     def setup_nodes(self):
         """Override this method to customize test node setup"""
-        extra_args = [[]] * self.num_nodes
-        if hasattr(self, "extra_args"):
-            extra_args = self.extra_args
-        self.add_nodes(self.num_nodes, extra_args)
+        self.add_nodes(self.num_nodes, self.extra_args)
         self.start_nodes()
         if self._requires_wallet:
             self.import_deterministic_coinbase_privkeys()